High-pressure decorative laminates have been widely employed in the building industry as counter and table tops, bathroom and kitchen work surfaces, furniture and cabinets, wall paneling and partitions and doors. Because they are durable and resistant to scratching and various liquids and heat, the high-pressure decorative laminates have been popular in the furniture industry, primarily as tops for furniture where they not only provide an attractive appearance matching adjacent natural wood finishes but also a surface more desirably durable than wood.
In the production of such furniture tops, it is necessary to cut or otherwise machine the top to the desired dimensions and configuration. This may be accomplished by simultaneously machining a stack of laminates which are thereafter individually bonded to a rigid structural base member or by first bonding a laminate sheet to a rigid base member of substantial thickness and then machining the composite. Irrespective of the method, the laminate must be machined. The hard durable surface provided by the laminate is relatively difficult to machine and carbide-tipped tools are frequently employed to reduce tool wear and minimize the down-time involved in sharpening tools. Machinability and tool wear are, accordingly, particularly important factors in the use of high-pressure decorative laminates for furniture because of the extensive machining that must be done. It is also a factor in most other applications because some machining is generally necessary.
Another property factor exists in handling such laminates, particularly large sheets, such as 5 .times. 12 foot sheets. A sheet that large and only 1/16 inch thick will deflect or bend considerably if not completely supported during handling. If not handled carefully, the large sheets may crack and thus render the entire sheet or a major portion thereof useless. It should be apparent that increasing the amount of deflection at rupture (flexural strength) would alleviate the handling problem. The foregoing problem is, of course, even more severe with thinner laminates such as the standard 1/32 inch thick laminates which are growing in commercial significance.
High-pressure decorative laminates are, of couse, themselves laminated articles comprising plural layers of synthetic resin impregnated paper sheets consolidated or bonded together into a unitary structure under heat and pressure. Conventionally, the decorative or print layer is a sheet of high quality purified alpha cellulose fiber and/or certain rayon fibers impregnated with a thermosetting condensation resin such as aminotriazine aldehyde resins, for example melamine formaldehyde resins. An overlay sheet, transparent when cured, may be employed to protect the decorative or print layer and is also a sheet of alpha cellulose, or the like, impregnated with an aminotriazine aldehyde. The overlay and print sheets are bonded to a plurality of core or body sheets of fibrous cellulosic material, usually kraft paper, most generally impregnated with a thermosetting phenol-formaldehyde resin. The major portion of the paper in a decorative laminate is composed of the core or body sheets rather than the print or overlay sheets. Typically seven or eight core sheets are consolidated with only a single print and single overlay sheet to form a conventional 1/16 inch decorative laminate.
Although the core sheets are less expensive than the print or overlay sheets, it is apparent that the core sheets are a significant cost factor, because of their volume in a decorative laminate. It is also apparent that many of the properties of the paper-base decorative laminates are derived from the papers employed as well as the resins employed. The properties of the core stock paper, then, will influence the properties of the end product decorative laminate.
Historically, in high-pressure decorative laminates, the core stock paper has been a relatively extensively cooked, low yield, high purity saturating grade kraft paper having a lignin content in the order of 2 to 5%, by weight. All percentages hereinafter expressed are weight percentages unless otherwise stated. The purpose of cooking is to dissolve the lignin and other noncellulose portions of the wood which bind the cellulose fibers together, thereby providing a pulp of free fibers which can be formed into a sheet of paper. Mixtures of hardwood and softwood species may be employed, the former providing relatively short fibers (0.5 to 2 mm. avg. length) and the latter providing relatively long fibers (2.5 to 5 mm. avg. length) in the paper. The paper must be well formed, have uniform texture, density and finish and be free of fiber bundles, hard lumps, large wood slivers, streaks, light areas and wrinkles. One of the most important properties of the paper is its absorbency. The paper must absorb a high percentage of resin to provide a satisfactory laminated product.
The properties of the laminated product, e.g. tensile strength, flexural strength, impact strength, the amount of absorption and thickness swell from immersion in boiling water and dimensional changes over an extreme range of relative humidity, are significantly influenced by the core stock paper. The laminated product should meet the standards set forth in NEMA Publication No. LD1-1971 for Laminated Thermosetting Decorative Sheets and the core stock papers must not preclude the obtention of the properties set forth in those standards.
The extensively cooked, low yield core stock papers are, of course, relatively high in cost. The advantages of a high yield paper for core stock in terms of lower relative cost and decreased pollution of water and air have provided the incentive for a considerable effort on the part of applicants and others to develop a high yield paper that could be used for the core stock of high-pressure decorative laminates. The most obvious direction, because of cost and the general and conventional desirability for long fibers in paper making, appeared to be a high yield kraft sheet made from a pulp containing a high proportion of soft or coniferous woods, particularly Southern pine species.
Working extensively with pulps containing in the order of 90% or more of the long fibered pine, a saturating core sheet was developed and successfully used to produce excellent high-pressure decorative laminates meeting all of the requirements set forth in the NEMA standards. Unexpectedly, we discovered that the laminates made from such core sheets caused severe wear and rapid deterioration of the carbide cutting tools conventionally employed in the normal fabrication into finished forms, particularly in furniture goods where extensive machining must be done. These core stock sheets, made from essentially all pine furnishes, at one point had relatively high lignin contents, in the order of fifteen (15) percent. We further discovered that the same pine furnishes, cooked to lower yield levels having lignin contents of 7-8 percent and below, produced laminates which met the NEMA standards, as one might then expect, but the tool degradation persisted to an unacceptable degree.